Near infrared optical interference filters with improved transmission

ABSTRACT

An interference filter includes a layers stack comprising a plurality of layers of at least: layers of amorphous hydrogenated silicon with added nitrogen (a-Si:H,N) and layers of one or more dielectric materials, such as SiO 2 , SiO x , SiO x N y , a dielectric material with a higher refractive index in the range 1.9 to 2.7 inclusive, or so forth. The interference filter is designed to have a passband center wavelength in the range 750-1000 nm inclusive. Layers of a dielectric material with a higher refractive index in the range 1.9 to 2.7 inclusive provide a smaller angle shift compared with a similar interference filter using SiO 2  as the low index layers.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Patent ApplicationSer. No. 62/107,112, filed Jan. 23, 2015, the entirety of which is fullyincorporated herein by reference.

FIELD OF USE

The following disclosure relates to the optical arts, optical filterarts, and related arts.

BACKGROUND

A known transmission interference filter employs a stack of alternatingsilicon and silicon dioxide (SiO₂) layers. Such devices are known foruse in the short wave and mid wave infrared down to about 1100 nm, asboth silicon and SiO₂ are transparent in this range. The lowerwavelength threshold (corresponding to the upper photon energythreshold) is controlled by the onset of absorption by the silicon,which in its crystalline form has a bandgap of about 1.12 eV. A keyadvantage of silicon in these devices is its high refractive index. Thespectral profile of an optical interference filter is, among otherthings, dependent on the angle of illumination. As the angles increasethe filters shift to shorter wavelength. This angular shift is dependenton the materials used and the distribution of those materials. Higherrefractive index results in less angle shift. For narrow band filtersthe amount of angle shift limits the useful bandwidth of the filter whenused in optical systems. In systems with large angular acceptance anglesa filter constructed such as to yield low angular shift can have anarrower passband and hence greater noise rejection than one constructedof materials with lower refractive index.

To extend device operation into the near infrared, it is further knownto hydrogenate the silicon, so as to employ alternating layers ofhydrogenated amorphous silicon (a-Si:H) and SiO₂. By hydrogenating thesilicon, the material loss and refractive index are reduced. By thisapproach, very high performance interference filters operating in the800-1000 nm range are achievable.

Some improvements are disclosed herein.

BRIEF SUMMARY

The disclosure relates to an interference filter comprising a stack of aplurality of layers of at least one layer of amorphous hydrogenatedsilicon and at least one layer of one or more dielectric materialshaving a refractive index lower than the refractive index of theamorphous hydrogenated silicon wherein the layers of one or moredielectric materials include layers of a dielectric material having arefractive index in the range 1.9 to 2.7 inclusive.

In some embodiments, the interference filter includes a stack of aplurality of layers of at least layers of amorphous hydrogenated siliconwith optimally added nitrogen (a Si:H,N) and layers of one or moredielectric materials, such as SiO₂, SiOx, SiOxNy, a dielectric materialwith a higher refractive index in the range 1.9 to 2.7 inclusive. Theinterference filter is designed to have a passband center wavelength inthe range 750 to 1000 nm inclusive. Layers of a dielectric material witha higher refractive index in the range 1.9 to 2.7 inclusive provide asmaller angle shift compared with a similar interference filter usingSiO₂ as the low index layers.

These and other non-limiting characteristics of the disclosure are moreparticularly disclosed below.

BRIEF DESCRIPTION OF THE DRAWINGS

The following is a brief description of the drawings, which arepresented for purposes of illustrating the exemplary embodimentsdisclosed herein and not for purposes of limiting the same. A preferredembodiment of the invention shall now be described with reference to theaccompanying Figures wherein:

FIG. 1 diagrammatically shows a sputter deposition system forfabricating near infrared optical interference filters with improvedtransmission and/or reduced angular shift as disclosed herein.

FIG. 2 diagrammatically shows the impact of hydrogenation on the opticalproperties (transmission and refractive index) of amorphous hydrogenatedsilicon (a-Si:H).

FIG. 3 diagrammatically shows the impact of nitrogen additive on theoptical properties (transmission and refractive index) of a-Si:H offixed hydrogenation level.

FIG. 4 diagrammatically shows an interference filter suitablymanufactured using the sputter deposition system of FIG. 1.

DETAILED DESCRIPTION

A more complete understanding of the processes and apparatuses disclosedherein can be obtained by reference to the accompanying drawings. Thesefigures are merely schematic representations based on convenience andthe ease of demonstrating the existing art and/or the presentdevelopment, and are, therefore, not intended to indicate relative sizeand dimensions of the assemblies or components thereof.

The present disclosure may be understood more readily by reference tothe following detailed description of desired embodiments and theexamples included therein. In the following specification and the claimswhich follow, reference will be made to a number of terms which shall bedefined to have the following meanings.

The singular forms “a,” “an,” and “the” include plural referents unlessthe context clearly dictates otherwise.

Numerical values in the specification and claims of this applicationshould be understood to include numerical values which are the same whenreduced to the same number of significant figures and numerical valueswhich differ from the stated value by less than the experimental errorof conventional measurement technique of the type described in thepresent application to determine the value.

All ranges disclosed herein are inclusive of the recited endpoint andindependently combinable (for example, the range of “from 800-1100 nm”is inclusive of the endpoints, 800 nm and 1100 nm, and all theintermediate values).

As used herein, approximating language, such as “about” and“substantially,” may be applied to modify any quantitativerepresentation that may vary without resulting in a change in the basicfunction to which it is related. The modifier “about” should also beconsidered as disclosing the range defined by the absolute values of thetwo endpoints. For example, the expression from about 2 to about 4″ alsodiscloses the range “from 2 to 4.” The term “about” may refer to plus orminus 10% of the indicated number.

The disclosure relates to near infrared optical interference filterswith improved transmission.

As previously noted, an interference filter comprising a stack of layerunits with hydrogenated silicon (a-Si:H) layers is used for operation inthe near infrared (800-1250 nm), because the hydrogenation of thesilicon decreases the absorption losses (both from intrinsic silicon anddisorder induced) sufficiently to provide acceptable filter transmissioncharacteristics in the passband. With brief reference to FIG. 2, it isrecognized herein that this approach for the near-infrared has asubstantial disadvantage. As seen in diagrammatic FIG. 2, for a fixedwavelength in the infrared (e.g. in the range 800-1100 nm), increasinghydrogenation of the a-Si:H (that is to say, increasing the hydrogencontent of the a-Si:H) does decrease the loss—however it also decreasesthe refractive index of the a-Si:H as diagrammatically depicted in FIG.2.

The performance of narrow band interference filters for high numericalaperture optical systems is a compromise between obtaining hightransmission with the low angle shift in the near infrared region wherethe material characteristics are changing rapidly. High transmissioncorresponds to low extinction coefficient (obtainable with high amountsof hydrogen) while small angle shift is achieved by high refractiveindex (obtainable with small amounts of hydrogen).

With brief reference to FIG. 3, the disclosed improvement pertains toadding a controlled amount of nitrogen to the a-Si:H layers of Si-basedinterference filters for use in the near-infrared (800-1100 nm). Saidanother way, this improvement entails substituting a-Si:H,N for a-Si:H.As diagrammatically shown in FIG. 3, for a fixed wavelength in theinfrared (e.g. in the range 800-1100 nm), and for a given (fixed) levelof hydrogenation, adding nitrogen increases the transmission with areduced concomitant reduction in refractive index. The impact of addingnitrogen on the refractive index is much less than the impact ofhydrogenation, especially for nitrogen fractions in the range of 10%nitrogen or lower. Accordingly, this modification enables fabrication ofnear-infrared interference filters operating in the range 800-1100 nmwith improved control of the angular shift, peak transmission and filterbandwidth.

On the other hand, for a given passband width, substituting a-Si:H,N fora-Si:H can provide improved transmission in the passband. In thisapproach, substituting a-Si:H,N for a-Si:H enables fabrication ofnear-infrared interference filters with improved transmission in thepassband as compared with equivalent a-Si:H-based device having the samerefractive index step (and hence the same spectral passband width).Indeed, the inventors have found that in this design paradigm thepractical operational range of such filters can be extended down to 750nm.

The skilled artisan will recognize that the spectral range encompassedby the disclosed a-Si:H,N based interference filters encompassespassbands of technological importance, such as the 850 nm optical datacommunications window.

In some interference filters applications operating in this range,another consideration is the angle shift of the passband. Conceptually,the angular shift results from the light ray path length through a layerincreasing with increasing angular deviation away from normal incidence.This increase in path length corresponds to a change in the phaseretardation, which affects constructive/destructive interference so asto introduce the angle shift. If the normal incidence path lengththrough a layer is d_(L), then the path length though the layer at anangle θ_(L) in the material (measured off the normal, i.e. θ_(L)=0 fornormal incidence) is d′_(L)=d_(L)/cos(θ_(L)). As θ_(L) is related to theangle-of-incidence θ of light impinging upon the interference filteraccording to Snell's law, and assuming the ambient is air (n=1), thisleads to θ_(L)=arcsin(θ/n_(L)) where n_(L) is the refractive index ofthe layer. Using the identity cos(u)=√{square root over (1−(sin(u))²)}enables this to be written as

$d_{L}^{\prime} = {d_{L}/{\sqrt{1 - ( \frac{\theta}{n_{L}} )^{2}}.}}$

It is thus seen that the angular shift effect is made worse by a smallrefractive index n_(L) of the layer.

In conventional interference filter design, it is typically desired tomaximize the refractive index contrast between the high index layers andthe low index layers. In silicon-based interference filters, the highrefractive index layers are a-Si:H (which could be replaced by a-Si:H,Nas disclosed herein) while silicon dioxide (SiO₂ having n˜1.4-1.5)serves as the low refractive index layers. However, it is disclosedherein to obtain reduced angular shift in interference filters operatingin the 750-1000 nm range by substituting a higher refractive indexmaterial for SiO₂ in some or all low index layers of the interferencefilter. In some contemplated embodiments, the substitute layer is adielectric layer that has a refractive index in the range 1.9 to 2.7inclusive. Some suitable Si-compatible materials providing these valuesinclude silicon nitride (Si₃N₄ having n−2.0-2.2), silicon oxynitride(SiO_(x)N_(y) with y large enough to provide a refractive index of 1.9or higher), tantalum pentoxide (Ta₂O₅ having n˜2.1-2.2), niobiumpentoxide (Nb₂O₅ having n˜2.3-2.4), or titanium dioxide (TiO₂ havingn˜2.6). In illustrative embodiments shown herein, silicon nitride(Si₃N₄) is used. The high index a-Si:H or a-Si:H,N layer should havehydrogen (and optionally nitrogen) content sufficient to provide thedesired refractive index contrast with the low index layers.

Moreover, to obtain a desired low angle shift for a design-specificationangle it may be sufficient to replace only some SiO₂ layers of the stackwith the higher index dielectric material (e.g. Si₃N₄). Optical designsoftware (e.g. a ray tracing simulator) can be used to optimize layerplacement and thicknesses for materials with known refractive index inorder to achieve desired center band, bandwidth, and angle shift designbasis characteristics.

With reference now to FIG. 1, a suitable manufacturing system isdescribed. The illustrative system employs sputter deposition—however,other deposition methods are contemplated, such as vacuum evaporation,electron-beam evaporation, or so forth. In general, either a.c. or d.c.sputtering may be used. The illustrative sputter deposition systemincludes a process chamber 10 containing a sputter target holder 12 anda substrate carousel 14. For the illustrative deposition, a silicontarget 16 (e.g. silicon wafer 16) is mounted in the sputter targetholder 12. One or more substrates 20 are loaded into the substratecarousel 14. The substrate(s) 20 are suitably of a material, such asglass, silica, or alumina, that is transparent in the wavelength rangeof interest (e.g. 800-1000 nm, or 750-1000 nm).

In sputter deposition, energetic particles are directed toward thetarget 16 (in this case a silicon target 16), which particles havesufficient energy to remove (i.e. “sputter”) material off the target,which then transfers (ballistically and/or under the influence of amagnetic or electric field) to the surface of the substrate(s) 20 so asto coat the substrates 20 with the sputtered material. The illustrativesputter deposition system employs argon (Ar) gas from an illustrative Argas bottle 22 or from another argon source as the energetic particles.An ionizing electric field generated by applying a negative bias (−V) tothe target 16 in order to ionize argon atoms which then bombard thenegatively biased target 16 under influence of the electric fieldgenerated by the −V voltage bias in order to produce the sputtering. Thesubstrate(s) 20, on the other hand, are biased more positively ascompared with the target 16, e.g. the substrate(s) 20 are grounded inthe illustrative sputter system of FIG. 1. In this illustrativeconfiguration, the target 16 is the cathode, and the chamber 10 (and/orthe substrate(s) 20, e.g. in some embodiments the substrate carousel 14may be grounded) is the anode, of an electric circuit. While argon isthe sputtering gas in the illustrative embodiment, other inert gasesthat can be ionized could be alternatively used, such as xenon.

To deposit silicon dioxide, an oxygen (O₂) bottle 24 or other oxygensource is provided. To deposit amorphous hydrogenated silicon withnitrogen additive (a-Si:H,N), a hydrogen (H₂) bottle 26 or otherhydrogen source (for example, ammonia, NH₄, or silane, SiH₄) and anitrogen (N₂) bottle 30 or other nitrogen source are provided. A(diagrammatically indicated) gas inlet manifold 32 is provided in orderto admit a desired gas mixture into the process chamber 10 during thesputter deposition process. Flow regulators 34 are adjustable to set theflow of Ar, O₂, H₂, and N₂, respectively. The process chamber 10 is alsoconnected with a suitable exhaust 36 (e.g. with scrubbers or the like)to discharge gas from the chamber 10. It is contemplated to substituteother gas sources for the illustrative O₂, H₂, and N₂ bottles. Othersuitable nitrogen gas sources include ammonia (NH₄) or hydrazine (N₂H₄).When using a gas source such as ammonia or hydrazine which includes bothnitrogen and hydrogen, calibrations should be performed to account forthe relative incorporation of nitrogen and hydrogen into the a-Si:H,Nlayer. Process parameters such as substrate temperature, target bias(−V), process chamber pressure, total flow rate, and so forth may impactrelative incorporate of nitrogen versus hydrogen. Two valves VA, VB areprovided to switch between depositing SiO₂ and a-Si:H,N. The valve VAcontrols admission of oxygen from the oxygen source 24 into the gasinlet manifold 32, while the valve VB controls admission of thehydrogen/nitrogen mixture from the hydrogen and nitrogen sources 26, 30.To enable rapid switching between SiO₂ deposition and a-Si:H,Ndeposition, the valves VA, VB are automated valves whose actuators arecontrolled by an electronic sputtering controller 40 in accordance witha filter recipe 42. For example, the sputtering controller 40 maycomprise digital-to-analog (D/A) converters, a high voltage source, anda microprocessor or microcontroller programmed to operate the D/Aconverters generate electrical actuation signals to open or closerespective valves VA, VB in accordance with the filter recipe 42 and tooperate the voltage source to apply the voltage −V to the target/cathode16. A lower right-hand inset table 50 shown in FIG. 1 summarizes thesettings for valves VA, VB to deposit SiO₂ and a-Si:H,N, respectively.To deposit SiO₂ the valve VA is open to admit oxygen to the gas inletmanifold 32 while the valve VB is closed to turn off the hydrogen andnitrogen sources. The resulting process gas is an argon/oxygen mixture.To deposit a-Si:H,N the valve VA is closed to block the oxygen and thevalve VB is opened to admit process gas comprising anargon/hydrogen/nitrogen mixture to the gas inlet manifold 32. Note thatthe argon source 22 is connected to the gas inlet manifold 32independently from the valves VA, VB. Separate manually operableshut-off valves (not shown) are typically provided for each gas source22, 24, 26, 30 to enable manual shut-off of each gas sourceindependently from the automatic valves VA, VB.

If it is further desired to substitute a higher refractive indexmaterial for some of the low index layers, additional gas sources may beprovided along with suitable valving. In the illustrative system of FIG.1, an additional nitrogen (N₂) bottle 25 or other nitrogen source areprovided, controlled by a valve VC, in order to deposit silicon nitride(Si₃N₄) layers. As further indicated in the table 50, deposition ofSi₃N₄ is obtained when valve VC is open and both valves VA and VB areclosed. As with deposition of SiO₂, the silicon component of the siliconnitride is supplied by the silicon-based sputtering target 20. Thedesired stoichiometry is set by the flow regulator on the nitrogenbottle 25 using suitable calibration runs. Although not shown in FIG. 1,it will be appreciated that a similar setup could be used to depositSiO_(x)N_(y) with refractive index of 1.9 or higher, by opening bothvalves VA, VC with valve VB closed. To substitute a dielectric layerthat does not contain silicon (e.g. Ta₂O₅, Nb₂O₅, or TiO₂), the targetholder 12 may have multiple target slots which are loaded with a silicontarget and also another slot loaded with a suitable target containing,e.g., tantalum, niobium, or titanium, for use in depositing thenon-silicon containing dielectric layers. Alternatively, the tantalum,niobium, titanium, et cetera may be provided by a gas source or othersource.

An illustrative interference filter fabrication process suitablyperformed using the fabrication system of FIG. 1 is described next.Initially, all gas sources 22, 24, 26, 30 are manually closed off, theprocess chamber 10 is brought to atmospheric pressure and opened, thetarget 16 is loaded onto the target holder 12, and the substrate(s) 20are loaded onto the substrate carousel 14. The process chamber 10 isthen closed and drawn down to a target vacuum level. As further setup,the flow regulators 34 are manually set to the desired flow rates.(Alternatively, it is contemplated for the flow regulators to be underautomatic control of the sputtering controller 40, in which case theregulators are suitably set in accordance with values provided in thefilter recipe).

Sputter deposition is initiated by flowing the appropriate process gasvia the gas inlet manifold 32 and applying the cathode bias −V to thetarget 16 in order to ionize Ar atoms which are driven by the electricfield to sputter silicon off the silicon target 16. The particularstartup sequence depends upon the particular sputter deposition systemand other design considerations: for example, in one approach theprocess gas flow is first initiated and then the cathode bias −V isapplied to initiate sputter deposition; alternatively, the bias can beapplied under an inert gas flow and sputter deposition initiated byadmitting the appropriate process gas.

During sputtering, valves VA and VB (and optionally VC) are opened andclosed in accord with the filter recipe 42 and the valve settings oftable 50 in order to alternate between depositing SiO₂ (and/oroptionally Si₃N₄) and a-Si:H,N layers. The layer thicknesses arecontrolled based on deposition time and a priori knowledge of depositionrates obtained from calibration depositions. Layer compositions aredetermined based on the process gas mixture controlled by the settingsof the flow regulators 34 which are set based on calibration depositions(such calibration deposition should also include process parameters suchas substrate temperature, target bias (−V), chamber pressure, and totalflow rate in the calibration test matrix, as such parameters may alsoimpact layer composition). After deposition of the stack of interferencefilter layers is completed, process gas flow and the bias voltage −V areremoved (again, the particular shutdown sequence depends upon theparticular deposition system and so forth), the process chamber 10 isbrought up to atmospheric pressure, opened, and the coated substrates 20are unloaded.

With reference to FIG. 4, a diagrammatic representation of a thuslyfabricated interference filter 100 is shown. The filter includes thesubstrate 102 (e.g. the glass, silica, or alumina substrate initiallyloaded onto the substrate carousel 14) and alternating layers ofa-Si:H,N 104 and SiO₂ 106 and/or Si₃N₄ 108. In the illustrativeinterference filter 100 the layer immediately adjacent the substrate 102is an a-Si:H,N layer 104, but in other embodiments a dielectric layermay be immediately adjacent the substrate. In the illustrativeinterference filter 100 the topmost layer is an a-Si:H,N layer 104, butin other embodiments a dielectric layer may be the topmost layer. Theillustrative stack includes an instance of immediately adjacentSiO₂/Si₃N₄ layers, which may be included if in accord with the design.The illustrative interference filter 100 includes layer stacks 110, 112on opposite sides of the substrate 102—to manufacture such a device, thesputtering chamber may need to be opened and the substrates “flippedover” on the substrate carousel 14. (Alternatively, the substratecarousel 14 may be configured to enable such a maneuver to be performedrobotically without breaking open the chamber). Such a filter with twofilter sides 110, 112 may, for example, be a passband filter in whichthe stack on one side is a high-pass filter and the stack on the otherside is a low-pass filter—a passband is then defined by a wavelengthrange that is both above the high pass filter cut-off and below thelow-pass filter cutoff.

A known application of this kind of filter is in applications usingsilicon detectors. These wavelengths are particularly useful in activedevices, in which a light source as well as a detector are present. Inthis spectral region, LEDs and lasers are readily available which areinexpensive, plentiful and efficient. Some major applications include,but are not limited to, infrared gesture controls of human-machine (e.g.computer) interaction, infrared night vision for automobiles, LIDAR,infrared night vision for security cameras and proximity CMOS sensorsused in mobile phone and elsewhere. In these applications the usefulwavelength is between 700 and 1100 nm. In this range the a-Si:H,N is ahigh index material suitable for optical applications. The typical indexin this range is 3.3˜3.5, whereas by comparison TiO₂ has refractiveindex of only about 2.3˜2.4. In some suitable embodiments, the a-Si:H,Nlayers includes between 2% and 8% hydrogen and between 3%˜7% nitrogenwith the balance being Si. In general, more hydrogen and nitrogencontents provide shorter wavelength operation. In general, nitrogenconcentrations as high as 6% to 12% are contemplated.

In the illustrative embodiments, the a-Si:H,N layers 104 alternate withSiO₂ layers 106. SiO₂ has advantageous properties for this purpose,including good chemical compatibility with a-Si:H,N and a low refractiveindex (n˜1.5) which provides a large refractive index step at theinterface with a-Si:H,N. However, it is contemplated to substituteanother dielectric layer for the SiO₂ layer. For example, the dielectricmay not have exact SiO₂ stoichiometry, e.g. the SiO₂ may be replaced bySiO_(x) where x is not precisely two (also referred to herein as“silicon suboxide”).

As another example, a silicon oxynitride (SiO_(x)N_(y)) layer iscontemplated as the dielectric layer in place of SiO₂. In general, whenadding nitrogen to go from SiO_(x) to SiO_(x)N_(y) the refractive indexof increases with nitrogen content: for example, stoichiometric siliconnitride (Si₃N₄) has a refractive index of about 2.0. However, a smallamount of nitrogen (that is, SiO_(x)N_(y) where x˜2 and x>>y) iscontemplated to improve interface quality between the a-Si:H,N layer 104and the adjacent dielectric layer. These compounds offer index tailoringthat permit the construction of novel material combinations andcontinuously varying refractive index profiles.

Some suitable design methods for designing the constitutent layerthicknesses the given refractive indices of the constituent layers arebased on the following. In general, the wavelength λ in the layer isgiven by λ=λ₀/n where λ₀ is the free space wavelength and n is therefractive index. Reflection from a surface of higher refractive indexintroduces a 180° phase shift, while no phase shift is introduced byreflection from a surface of lower refractive index. Using theseprinciples and given the refractive indices of the constituent layers,the thicknesses of the constituent layers are chosen such that, for thedesign-basis passband center wavelength, the optical path lengthsthrough each layer and reflected at its interface with the next layerconstructively combine, that is, are integer multiples of thewavelength. More elaborate interference filter design techniques forchoosing the constituent layer thicknesses (and refractive indices ifthese are also optimized parameters) are given in: H. Angus Macleod,THIN-FILM OPTICAL FILTERS, FOURTH EDITION (Series in Optics andOptoelectronics, CRC Press 2010).

While the illustrative interference filters include repeating units oftwo layers, it is contemplated to incorporate three or more layers intothe repeating unit, such as an a-Si:H,N layer and two differentdielectric layers, to achieve desired passband properties (e.g. centerwavelength, FWHM, “flatness” of the passband, et cetera).

The present disclosure has been described with reference to exemplaryembodiments. Obviously, modifications and alterations will occur toothers upon reading and understanding the preceding detaileddescription. It is intended that the present disclosure be construed asincluding all such modifications and alterations insofar as they comewithin the scope of the appended claims or the equivalents thereof.

Further non-limiting disclosure is set forth in the followingone-sentence statements formulated as patent claims.

1. An interference filter comprising: a layers stack comprisingplurality of layers of at least: layers of amorphous hydrogenatedsilicon and layers of one or more dielectric materials having arefractive index lower than the refractive index of the amorphoushydrogenated silicon wherein the layers of one or more dielectricmaterials include layers of a dielectric material having a refractiveindex in the range 1.9 to 2.7 inclusive.
 2. The interference filter ofclaim 1 wherein the layers of a dielectric material having a refractiveindex in the range 1.9 to 2.7 inclusive include one or more layerscomprising Si₃N₄, SiO_(x)N_(y) with y large enough to provide arefractive index of 1.9 or higher, Ta₂O₅, Nb₂O₅, or TiO₂.
 3. Theinterference filter of claim 2 wherein the layers of one or moredielectric materials further include SiO₂ layers.
 4. The interferencefilter of claim 1 wherein the layers stack is configured to have apassband center wavelength in the range 800-1100 nm inclusive.
 5. Theinterference filter of claim 1 wherein the layers stack is configured tohave a passband center wavelength in the range 750-1100 nm inclusive. 6.The interference filter of claim 1 further comprising: a transparentsubstrate supporting the layers stack.
 7. The interference filter ofclaim 6 wherein the transparent substrate comprises a glass substrate.8. The interference filter of claim 6 wherein the layers stack includesa first layers stack on one side of transparent substrate and a secondlayers stack on the opposite side of the transparent substrate.
 9. Theinterference filter of claim 8 wherein the first layers defines a lowpass filter with a low pass cutoff wavelength, the second layers stackdefines a high pass filter with a high pass cutoff wavelength, and theinterference filter has a passband defined between the high pass cutoffwavelength and the low pass cutoff wavelength.
 10. A method comprising:fabricating an interference filter as set forth in claim 1 by operationsincluding depositing the layers stack on a substrate.
 11. The method ofclaim 10 wherein the depositing comprises sputtering using at least asilicon-based sputtering target.
 12. An interference filter comprising:a layers stack comprising plurality of layers of at least: layers ofamorphous hydrogenated silicon and layers of one or more dielectricmaterials having a refractive index lower than the refractive index ofthe amorphous hydrogenated silicon including layers of a dielectricmaterial having a refractive index in the range 1.9 to 2.7 inclusive.13. The interference filter of claim 12 wherein the layers of adielectric material having a refractive index in the range 1.9 to 2.7inclusive include one or more layers comprising Si₃N₄, SiO_(x)N_(y) withy large enough to provide a refractive index of 1.9 or higher, Ta₂O₅,Nb₂O₅, or TiO₂.
 14. The interference filter of claim 12 wherein thelayers of one or more dielectric materials further include SiO₂ layers.15. The interference filter of claim 14 wherein the layers stackincludes at least one SiO₂ layer immediately adjacent a layer of adielectric material having a refractive index in the range 1.9 to 2.7inclusive with no intervening layer of amorphous hydrogenated silicon.